Alkaline phosphatase (ALP) is the earliest known marker of osteoblastic differentiation. The proposed studies use ALP expression to investigate mesenchymal-osteoblastic differentiation during rat mandibular development. Osteogenesis in the developing mandible will be studied by correlation of developmental events such as preosteogenic mesenchymal condensation and osteoid deposition with ALP expression at the protein and mRNA levels (visualized by in situ hybridization). Additional experiments will correlate these events with the expression of type I collagen. We will further investigate molecular mechanisms involved in osteoblast-specific regulation of ALP expression. These mechanisms include differential utilization of alternative promoters and alternative 5' exons of the ALP gene in different tissues and during osteoblastic differentiation, and transcriptional control by cis-regulatory DNA sequences in the ALP gene, particularly bone specific regulatory elements in this gene. These studies will improve our understanding of normal craniofacial development and osteoblastic differentiation, and will allow future studies of pathogenic processes which affect craniofacial development.